The present invention relates to a resonator structure, in which the resonator is provided in an insulating material with said resonators.
A resonator is known in the art in which a conductive strip is provided on an insulating material, the length of said strip being half-wave or quarter-wave, whereby both ends of the strip are grounded or only one of the ends is grounded while the other is open. The insulating material is usually a circuit board in which the surface opposite to the strip carrying surface is metallized and forms a ground plane. Also the surface to the strip around the strip can be metallized so that a narrow non-conductive area is left between the strip and the metallized surface. The structure is known as microstrip structure in the art. The strip can be placed on a separate piece away from the circuit board, such as a ceramic bit, said bit being mountable on the circuit board. An advantage of the structure lies therein that a substrate provided with high quality electrical properties can be used for the strip line base, and for the circuit board material, a material with less powerful electrical properties can be used which is easier to work and which is less costly.
When an insulating layer and a ground plane are placed on both sides of the resonator strip, a strip line resonator is in question. An aberrant strip line resonator is described in U.S. Pat. No. 4,785,271. The resonator structure therein disclosed consists of two dielectric substrates with a resonator in the middle thereof, the cross-section thereof being elliptic or rectangular. This has been so produced that on the non-conductive surface of the substrate a groove has been made by milling or otherwise in which the cross-section is an elliptical curve or a rectangle.
The intact areas remain between both ends of the groove and the edge of the substrate surface, i.e. groove, does not extend across the entire surface. The groove has been coated with a conductive layer, and at a given point of the groove a strip line on the plane surface defining the groove is connected, one end of said strip line being on the edge of the surface. Said strip line serves as the input line for the signal, Or,as the line for the output signal. When two of such substrate pieces are connected by filling the grooves with an appropriate adhesive and by placing the grooves against each other, a strip line resonator is produced in which the central conductor is not a strip line but a tube with e.g. elliptical cross-section. The "tubular" structure of the central conductor reduces its impedance because the local increase in the current density caused by the sharp edges of the conventional strip line has been omitted.
The Finnish Patent Application No. 922101, filed at the same time with the present application, included as reference to thereto, discloses a strip line resonator in which a conductive strip has been immersed inside the plane of the dielectric substrate surface and the conductive strip itself has been produced by coating the surface of the groove produced on the substrate with a conductive material. The other surfaces of the substrate have been coated, with the exception of the surface provided with the groove, with a conductive material, acting as a ground plane. The groove extends from edge to edge of the substrate across the entire surface.
The resonator according to said Finnish patent application is composed of a rod-like piece of dielectric material, preferably ceramic material, the cross-section thereof being rectangular, as seen in the end face 3. The piece comprises an upper surface, a lower surface and the side surfaces. A groove 7 has been produced on the upper surface, extending in parallel with the longer side of the surface throughout the entire surface from the end 3 to the opposite end, dividing the upper surface into two surface parts 5 and 6. All surfaces, except the upper surface parts 5 and 6, are coated with an electrically conductive material, e.g. silver-copper blend. The surfaces may also be left uncoated, and some other conductive layer can be used around the structure, e.g. a metallic housing. Also the surface of the groove 7 has been coated in the same process. The coating of the groove is at least at one edge 8 beer connected with the coating of the end face. If the surface 3 has been coated, a narrow uncoated area 11 can be produced in the opposite end of the groove, whereby no electrically conductive connection between the coating of the groove and the coating of the end 3 exists. The coating of the groove may also be connected directly to the coating of the end face 3. The end face 3 may also be uncoated so that no distinguishing area 11 is needed. Thus, the groove 7 forms a transmission line resonator of the length of quarter-wave or half wave being dependent on whether only one end or both ends of the groove is/are connected to the coating of the end.
A grooved ceramic piece tan be made by any process known in the art, such as dry pressing, extrusion moulding or injection moulding. Also a piece of plate may be used into which a groove is cut.
A disadvantage of a strip line resonator is that the possibilities to tune the electrical properties of the resonator are quite poor due to the sandwich structure. On the other hand, the Q values thereof as well as the Q values of a coaxial resonator are good. In contrast, tuning with a microstrip structure is easy, but the quality factors, i.e. the Q values, are insufficient in some applications. It is true that in a groove resonator disclosed in said Finnish application better Q values can be obtained than in the microstrip resonator, but for certain applications a resonator with even higher Q values is required, but which should be simple to make, easy to tune, and which can be made thinner than e.g. a coaxial resonator.